1. Field of the Invention
This invention relates to a gas detector and more particularly to a gas leak detector with improved reliability in detecting the leak of fuel gases, such as a hydrogen gas, methane gas, butane gas.
2. Prior Art
Generally, metallic oxide semiconductors, such as SnO.sub.2, ZnO, Fe.sub.2 O.sub.3, In.sub.2 O.sub.3, WO.sub.3, CeO.sub.2, have a property to change their resistance when they come into contact with hydrogen gas, methane gas, butane gas, etc., while they are heated to high temperature. Therefore, by using said property, gas sensing element (gas sensor) to detect the leakage of fuel gases, such as LPG (liquified petroleum gas), natural gas, have been put in practical use. However, said gas identification elements are defective in selectivity to gases. That is, they have the property to show the change in resistance not only to the target gases to be detected, such as hydrogen gas, methane gas, butane gas, contained in LPG, natural gas, etc. which are consumed domestically, but also to ethanol gas, steam, which are formed during cooking. As a result, they detect also the non-target gases in addition to the target gases to be detected, thereby degrading the reliability of detection.
Consequently, a gas leak detector with highly reliable detecting performance, that is not only capable of detecting the target gases in presence solely of said target gases, such as hydrogen gas, methane gas, butane gas, but also capable of detecting said target gases in concurrent presence non-target gases which are not to be detected, such as steam, ethanol gas, smoke, without getting disrupted by the non-target gases, and furthermore, that does not transmit the detection signal to alarm circuit when the non-target gases alone are present, thereby avoiding wrong warning.
In conjunction with gas leak detection, following prior art has already been disclosed. That is, U.S. Pat. No. 3,644,795 discloses a structure, wherein highly strong gas detection elements are used by obtaining them through adding silicon compound into gas sensor components including semiconductors of metallic oxide, such as SnO.sub.2, ZnO, Fe.sub.2 O.sub.3, NiO or Cr.sub.2 O.sub.3, and the output induced by change in resistance of said elements is input to a buzzer. U.S. Pat. No. 3,835,529 discloses a method for preparing a gas sensing element composed of metallic oxide semiconductors, such as SnO.sub.2, ZnO, Fe.sub.2 O.sub.3, TiO.sub.2, Cr.sub.2 O.sub.3, NiO, CoO, through processes of mixing, forming, baking and installation of electrode. And in U.S. Pat. No. 3,732,519, a gas sensing element including a pair of electrodes and porous metallic oxides containing semiconductor particles, wherein the metallic oxides contain the particles of Al.sub.2 O.sub.3 and SiO.sub.2, was disclosed. However, said prior art is incapable of achieving the purpose to give the warning only when the fuel gas leakage takes place.